Redox Behaviour and Redox Potentials of Dyes in Aqueous Buffers and Protic Ionic Liquids

Organic dyes hold promise as inexpensive electrochemically-active building blocks for new renewable energy technologies such as redox-flow batteries and dye-sensitised solar cells, especially if they display high oxidation and/or low reduction potentials in cheap, non-flammable solvents such as water or protic ionic liquids. Systematic computational and experimental characterisation of a representative selection of acidic and basic dyes in buffered aqueous solutions and propylammonium formate confirm that quinoid-type mechanisms impart electrochemical reversibility for the majority of systems investigated, including quinones, fused tricyclic heteroaromatics, indigo carmine and some aromatic nitrogenous species. Conversely, systems that generate longlived radical intermediates - arylmethanes, hydroquinones at high pH, azocyclic systems - tend to display irreversible electrochemistry, likely undergoing ring-opening, dimerisation and/or disproportionation reactions. Redox-active protic ionic liquids hold promise as low-cost, high energy density electrolytes for redox flow batteries. Their redox behaviour is similar to parent salts in pH-neutral aqueous buffer, although they may undergo different side reactions. In this work, fortuitous side-reactions yield quinone-imines which display promising electrochemistry for flow battery applications. image